Erase and program verification circuit for non-volatile memory

ABSTRACT

For non-volatile memory devices, such as flash EPROM integrated circuits, which have memory cells and reference cells, and sense circuitry responsive to addressed memory cells and the reference cells, and in which a read potential is supplied to the gate of the selected memory cells and a reference potential is supplied to the gate of a reference memory cell during a read mode, the state of the programmable memory cells is verified by (1) supplying a first verify potential to the gate of an address programmable memory cell; and (2) supplying a second verify potential to the gate of the reference cell which is different from the first verify potential. Because cell current is a very strong function of the gate voltage, applying different gate voltages to the memory and reference cells is equivalent to adjusting the sense ratio. When the method is applied for program verify, then the second verify potential applied to the reference cell is less than the first verify potential applied to the addressed programmable memory cell. When the method is applied for erase verify, the second verify potential is greater than the first verify potential.

This application is a divisional of application Ser. No. 08/108,670,Filed Aug. 31, 1993 now U.S. Pat. No. 5,463,586.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the design of erasable and programmablenon-volatile memory devices; and more particularly to circuits forverifying a programmed or erased state of memory cells in the device,suited for flash EPROM or EEPROM memory cells.

2. Description of Related Art

Non-volatile memory design based on integrated circuit technologyrepresents an expanding field. One popular class of non-volatile memorycell is known as the erasable-programmable read only memory (EPROM). Twopopular EPROM designs are distinguished in the manner in which erasureof the memory cells is carried out. The first is referred to as theEEPROM which uses an electrical erasure routine that involves relativelyhigh voltage. A second member of this class is known as the flash EPROMwhich uses a lower voltage erasing technique.

Both the flash EPROM and EEPROM technologies are based on a memory cellwhich consists of a source, channel, and drain with a floating gate overthe channel and a control gate isolated from the floating gate. The actof programming the cell involves charging the floating gate withelectrons, which causes the turn on threshold of the memory cell toincrease. Thus, when programmed the cell will not turn on, that is itwill remain non-conductive, when addressed with a read potential appliedto its control gate. The act of erasing the cell involves removingelectrons from the floating gate to lower the threshold. With the lowerthreshold, the cell will turn on to a conductive state when addressedwith a read potential to the control gate.

Both the flash EPROM and EEPROM memory cells suffer the problem ofover-erasure. Over-erasure occurs if, during the erasing step, too manyelectrons are removed from the floating gate leaving a slight positivecharge. This biases the memory cell slightly on, so that a small currentmay leak through the memory cell even when it is not addressed. A numberof over-erased cells along a given bit line can cause an accumulation ofleakage current sufficient to cause a false reading. The regular EEPROMdesign uses a two transistor cell structure which includes a pass gatethat isolates the memory cell from the bit line, so that unselectedmemory cells do not contribute leakage current to the bit line. Theflash EPROM cell does not use the isolation transistor, so over-erasurecauses a significant problem in the flash EPROM design.

Over-erasure also illustrates an important phenomenon involved with theprogramming and erasing of floating gate memory cells. That is, theamount of charge which is moved into the floating gate during a givenprogramming phase or moved out of the floating gate during a givenerasure phase cannot always be tightly controlled. This amount of chargedepends on such factors as the temperature of the cell at the time ofthe operation, variations in the cells which occur due to processingtechnology, cell aging, and other factors.

Therefore, commercial flash EPROM designs include circuitry forverifying the success of programming and erasing steps. See, forinstance, U.S. Pat. No. 4,875,788, entitled VOLTAGE MARGINING CIRCUITFOR FLASH EPROM, invented by Jungroth. The prior art devices include afirst mode for verifying the programming of the cell during which thepotential supplied to the control gate of the cell (across word lines inthe memory array) is increased above the normal read potential. Thus,the Jungroth patent provides for applying a 5 V potential to the cellfor normal read operations, and a higher potential of approximately 7.5V during the program verify. By performing program verify with a highervoltage on the control gate, the circuit ensures that the programmingstep resulted in injection of a sufficient number of electrons into thefloating gate to raise the turn on threshold with a safe margin over theminimum amount required. Similarly, during erase verify, the voltage onthe control gate is reduced by Jungroth to approximately 3.25 V insteadof 5 V. If the cell conducts with 3.25 V applied to its control gate,then it will surely conduct if the read potential of 5 V is applied.Again, this ensures removal of a sufficient amount of charge from thefloating gate with a significant margin for safety over the minimumrequired removal for successful erase.

The standard sensing technology applied to flash EPROM cells involves adifferential sense amp which has one input connected to a bit line of aselected cell, and a second input connected to the bit line of areference cell. The effective resistive load R1 on the bit line of theselected cell and the effective resistive load R2 on the bit line of thereference cell have an effect on the sensing operation known as thesense ratio. Thus, the ratio of R1/R2 determines the ratio of current onthe selected cell to current on the reference cell which triggers thesense amp to indicate a conductive state of the memory cell. Forinstance, a sense ratio of 2.5 will require a current level on theselected bit line of 40% of the current on the reference bit line toindicate a conductive state of the cell.

Prior art systems for verifying programming and erasing, change thelevel of voltage applied to the control gates of both the memory andreference cells together. To provide further margin, the prior artsystems have manipulated the load on the bit lines to the sense ratio byincreasing the sense ratio from approximately 2.5 to say approximately 4during program verify. With a sense ratio of 4, a lower level of currenton the selected bit line (25% of the current on the reference bit line)is required to trip the sense amp. Similarly, reducing the sense ratiobelow 2.5 during erase verify is used to increase the amount of currentrequired on the selected bit line to trip the sense amp.

While prior art designs have used a combination of voltage margining onthe word lines and sense ratio adjustment based on increasing ordecreasing the resistive load of the bit lines, these systems continueto have certain disadvantages. In particular, it is difficult to finelyadjust the load of the bit lines to control the sense ratio. This can bedone by switching transistors on and off on the load to reduce orincrease the resistance. However, the granularity of such techniques israther crude.

Accordingly, there is a need for an erase verify and program verifycircuit for flash EPROMs and other non-volatile memory cells, whichallows for finer control of the verify margins involved.

SUMMARY OF THE INVENTION

The present invention provides a technique for much finer control overthe design of verify circuits for non-volatile memory devices. Also, theverification potentials are also much closer to the normal read voltage.For non-volatile memory devices which have memory cells and referencecells, and sense circuitry responsive to addressed memory cells and thereference cells, and in which a read potential is supplied to the gateof the selected memory cells and the reference potential is supplied tothe gate of a reference memory cell during a read mode, the inventioncan be characterized as a method for verifying the state of theprogrammable memory cells which comprises:

supplying a first verify potential to the gate of an addressedprogrammable memory cell; and

supplying a second verify potential to the gate of the reference cellwhich is different from the first verify potential.

Because cell current is a very strong function of the gate voltage,applying different gate voltages to the memory and reference cells iseffectively equivalent to adjusting the sense ratio. However, thegranularity of the adjustments available are much finer. Also, thetechnique can be applied to systems with or without circuits foraltering the load on the selected and reference bit lines to control thesense ratio during the verify modes.

According to one aspect of the method, the reference potential and theread potential which are applied to the cells during the read mode aresubstantially equal. When the method is applied to verify the state ofprogrammable memory cells which is not conductive in response to a readpotential (program verify), then the second verify potential applied tothe reference cell is less than the first verify potential applied tothe addressed programmable memory cell.

When the method is applied to verify state of a programmable memory cellwhich is conductive in response to a read potential (erase verify), thesecond verify potential is greater than the first verify potential.

The invention can also be characterized as a programmable non-volatilememory device which includes an array of memory cells, and sensecircuitry having a reference cell. The sense circuitry detects the stateof an addressed memory cell with reference to the output of thereference cell. A voltage supply circuit supplies energizing voltage tothe control terminals of the selected memory cells and to the referencecell, to enable the difference in outputs from the selected memory celland the reference cell to indicate a state of the selected memory cell.Control circuitry is provided for carrying out the method describedabove which includes a read mode during which energizing voltage for thememory cells in the array is at a read potential and the energizingvoltage of the reference cell is at a reference potential, and a verifymode in which the energizing voltage for the array is a first verifypotential, and the energizing voltage for the reference cell is a secondverify potential different from the first verify potential.

The device may include memory cells which consist of flash EPROM cellsin a preferred system. Alternatively, the memory cells may consist ofelectrically erasable EPROM cells, or other related non-volatile cells.

According to yet another aspect of the present invention, a flash EPROMintegrated circuit is provided. The integrated circuit includes a memoryarray of flash EPROM memory cells having gates, sources, and drains. Aplurality of bit lines traverse the memory array, each bit line coupledto the drains of a column of cells in the array. A plurality of wordlines similarly traverse the memory array, with each word line coupledto the gates of a row of cells in the memory array. The integratedcircuit includes a reference array which includes at least one column offlash EPROM memory cells having gates, sources, and drains. A referencebit line is coupled to the drains of a reference column of cells in thereference array. Sense circuitry is coupled to the plurality of bitlines in the memory array and the reference bit line, for detecting astate of a selected memory cell in response to the state of the bitlines.

According to the present invention, the integrated circuit also includesa controllable voltage source which is coupled to the word lines in thememory array, and to at least one cell in the reference column in thereference array, for supplying a read energizing potential to selectedword lines in the memory array and a reference energizing potential to agate of at least one cell in the reference column of the referencearray. The controllable voltage source has a read mode, an erase verifymode, and a program verify mode, during which the read energizingpotential and the reference energizing potential may be independentlyset. In the read mode, the read energizing potential has a firstparticular level and the reference energizing potential has a secondparticular level, which may be substantially equal to the firstparticular level. In the erase verify mode, the read energizingpotential has a third particular level and the reference energizingpotential has a fourth particular level, and in which the thirdparticular level is less than the fourth particular level. During theprogram verify mode, the read energizing potential has a fifthparticular level and the reference energizing potential has a sixthparticular level and in which the fifth particular level is greater thanthe sixth particular level.

By controlling the energizing potential on the memory array and thereference array separately, in a system in which the sense circuitry ischaracterized by a sense ratio which is inversely proportional to anamount of current in a selected bit line relative to an amount ofcurrent in the reference bit line which indicates a conductive state ofa selective cell, the third and fourth particular levels are selected toeffectively lower the sense ratio during the erase verify mode byapplying lower energizing potential to the selected cell than to thereference cell. Similarly, the fifth and sixth particular levels areselected to effectively raise the sense ratio during the program verifymode by applying higher energizing potential to the selected memory cellthan to the reference cell.

According to a further aspect, the integrated circuit according to thepresent invention, includes an input for supplying a program potentialV_(PP) which is substantially 12 V±0.6 V, as well as a normal potentialV_(DD) which is substantially 5 V±0.5 V. The controllable voltagesource, according to the present invention, uses a voltage dividerderived from the V_(PP) potential which has a less percentage variationthan the V_(DD) potential. This results in less variation in the verifymode potentials applied to the gates of the transistors.

According to another aspect, the voltage divider is composed of a seriesof p-channel MOS transistors connected in a diode configuration.Further, the p-channel transistors are formed in n-wells, with then-well coupled to the source terminal. This achieves a stable voltagedivider consuming less die area and consuming less power than prior artresistance dividers.

According to yet another aspect, the voltage applied to the word linesin the memory array is supplied through a controllable voltage driverwhich supplies a read potential, a program verify potential, and anerase verify potential to a selected word line in the memory cell array.This driver includes an n-channel MOS transistor which has a thresholdvoltage that affects the output level of the driver. In this aspect, thevoltage divider based on the series connected p-channel transistors,includes an n-channel compensation transistor in series with thep-channel transistors. This way, variations in the threshold of then-channel transistor in the word line driver are offset in the voltagedivider so that the verify voltage does not depend on the n-channeltransistor threshold voltage variations.

According to yet another aspect of the present invention, thecontrollable voltage source according to the present invention can beapplied to an integrated circuit in combination with a circuit forcontrolling the load on the bit lines of the memory cell and thereference cell independently. Thus, the sense ratio can be directlyadjusted by adjusting the resistive load on the bit lines, incombination with the effective adjustment accomplished by varying theenergizing potentials to the gates of the memory and reference cells,respectively.

The present invention provides a flexible and efficient design forimplementing erase and program verify modes on non-volatile memorydevices, such as flash EPROM integrated circuits. The design providesfor precise control of energizing potential to the word lines of thememory array and the reference array, and independent control of thelevels of such energizing potentials for the read, erase verify, andprogram verify modes. The design is easily adjusted to provide forbuilt-in margin voltages with very fine granularity of control overmargins applied.

Other aspects and advantages of the present invention can be seen uponreview of the figures, the detailed description, and the claims whichfollow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic block diagram of an integrated circuit,non-volatile memory device according to the present invention.

FIG. 2 is a schematic diagram of a section of a memory array accordingto a preferred embodiment of the present invention.

FIG. 3 is a schematic diagram of a section of a reference arrayaccording to a preferred embodiment of the present invention.

FIG. 4 is a schematic diagram of the AVX generator for the system ofFIG. 1.

FIG. 5 is a schematic diagram of the margin voltage generator for thesystem of FIG. 1.

FIG. 6 is a schematic diagram of the word line driver for the system ofFIG. 1.

FIG. 7 is an expanded block diagram of the reference mini-array for thesystem of FIG. 1.

FIG. 8 is a schematic diagram of the bit line load circuitry for thereference bit line involved in the sense ratio parameter for the systemof FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed description of preferred embodiments is provided with respectto FIGS. 1-8. FIG. 1 provides a simplified schematic diagram of anintegrated circuit flash EPROM device according to the presentinvention. FIG. 2 illustrates one preferred embodiment of the flashEPROM memory array architecture and cell structure. FIGS. 3-8 illustrateimplementations of the primary functional blocks of FIG. 1 for a systemusing the memory cell structure of FIG. 2.

As shown in FIG. 1, an integrated circuit, non-volatile memory deviceincludes a memory array 10 which includes a plurality of rows andcolumns of memory cells. The columns are defined by a plurality of cellswhich are coupled to a bit line 11. The rows are defined by pluralitiesof cells which are coupled to a word line 12. In the figure, a singlebit line 11, and a single word line 12 are illustrated for the purposeof clarity, but are representative of the word lines and bit lines forthe entire array. It will be understood that the architecture for thememory 10 can take on a variety of structures. A preferred system mayinclude the architecture specified in our co-pending U.S. patentapplication entitled NON-VOLATILE MEMORY CELL AND ARRAY ARCHITECTURE,Ser. No. 07/823,882, filed Jan. 22, 1992, now abandoned, which was ownedat the time of filing and is currently owned by the same Assignee as thepresent application.

The memory array 10 includes non-volatile memory cells which arecharacterized by floating gate structures, such as flash EPROM cells orEEPROM cells. Thus, the integrated circuit includes erase and programdrivers 13 which are coupled, as schematically illustrated, across line14 to the memory array 10. The erase and program drivers receive aprogramming potential V_(PP) on line 15 from either on or off chipcircuitry, as appropriate, for the purposes of establishing the highvoltages needed in the erase and program operations. Also, controlsignals 16 are supplied to the erase and program drivers 13 to establishthe appropriate states for the program and erase functions.

Cells within the memory array 10 are selected using a column decoder 17and a row decoder 18, which are responsive to the address 19 which issupplied to the chip. The column decoder 17 generates the signal Y online 20 which drives a column select transistor 21 on each bit line 11.

The row decoder 18 controls word line drivers 22 which generate avoltage V_(WL) on the word line 12 for a selected row of the memoryarray 10.

The integrated circuit also includes a reference mini-array 23 whichincludes at least one column of memory cells coupled to a reference bitline 24. A supply potential V_(DD) is supplied on line 25 to the circuitin general, and the mini-array 23, and coupled during normal readoperations to a word line in the reference mini-array 23. A columnselect transistor 26 for the reference bit line 24 is left in an oncondition by charging its gate to the potential V_(DD).

The state of a selected memory cell in the memory array 10 is detectedby sense circuitry 27 which detects differences in the amounts ofcurrent flowing on the bit line 11 and on the reference bit line 24 asindicated by the voltages at nodes SA1 and SA2, respectively. The memorybit line 11 includes an effective resistive load R1 from the node SA1 tothe power supply V_(DD). Similarly, the reference bit line 24 includesan effective resistive load R2 from node SA2 to the power supply V_(DD).The resistors R1 and R2 shown in FIG. 1 schematically represent theresistive load of the bit line circuitry which may be based on loadtransistors or resistors or a combination of devices as suits aparticular design.

The ratio of the resistance R1 to the resistance R2 defines a parameterknown as the sense ratio SR for the sense circuitry 27, because thevalues of these resistances are determinative of a trip current for thememory bit line 11 which will cause the sense amplifier 27 to indicatean output on line 28 corresponding to a conductive state of a selectedmemory cell in the array 10. The sense ratio for the present applicationis defined as the ratio of resistance R1/R2. For a sense ratio of 2.5,the trip current is defined as 1/2.5 of the current on reference bitline 24, or 40% of the current on the reference bit line 24. Thus, ifbelow 40% of the reference current is flowing on bit line 11, anon-conductive state is indicated by the sense circuitry 27. If greaterthan 40% of the reference current is flowing on the memory bit line 11,then a conductive state of the memory cell is indicated by the sensecircuitry 27.

As mentioned above, the erase and program operations which are executedunder control of the erase and program drivers 13 are followed byrespective erase verify and program verify operations.

According to the present invention, the voltage V_(WL) on the word line12 has a first particular value during a normal read operation which issubstantially V_(DD), and a cell in the reference mini-array 23 isenergized by a second particular level, which is again substantially thevoltage V_(DD) on line 25 in the embodiment described. The first andsecond particular values may or may not be equal, as suits the needs ofa particular design.

During the erase verify mode, the voltage V_(WL) on line 12 has a thirdparticular value, and a cell in the reference mini-array 23 is energizedwith an erase verify source voltage on line 29 which has a fourthparticular value EVS.

In the program verify mode, the voltage V_(WL) on line 12 has a fifthparticular value. Also, a cell in the reference mini-array 23 isenergized with a program verify source voltage on line 30 which has asixth particular value PVS.

The voltage on the word line 12 is driven through word line driver 22 inresponse to the source voltage AVX on line 31. Line 31 is driven by theAVX generator 32. The AVX generator 32 receives as inputs the V_(DD)potential 25, the V_(PP) potential 15, and control signals 16. Also, theAVX generator 32 receives controlled reference voltages PV1, PV2, EV1,and EV2 across lines 33 from a margin voltage generator 34. The inputsto the margin voltage generator 34 include V_(DD) 25, V_(PP) 15, andcontrol signals 16. Also, the margin voltage generator 34 supplies thePVS voltage on line 30 and the EVS voltage on line 29.

The AVX generator 32 supplies substantially V_(DD) on line 31 during theread mode. During program verify, the voltage AVX is generated inresponse to the values PV1 and PV2 so that an energizing level on theword line 12 higher than V_(DD) is established. During erase verify, thevoltage AVX is generated in response to EV1 and EV2 so that a voltagelevel on the word line 12 is established which is less than V_(DD).

During the normal read mode, the V_(DD) signal on line 25 energizes acell in the reference mini-array 23 so that the cell in the referencemini-array and the selected memory cell and the memory array 10 areenergized by substantially the same gate voltage. In this mode, thesense ratio is effectively set by the resistances R1 and R2.

During erase verify, the EVS signal on line 29 is energized and iscoupled to a cell in the reference mini-array 23, while V_(DD) and PVSare disconnected from the reference mini-array word lines. In this mode,the voltage EVS 29 is higher than the energizing voltage on the wordline 12.

Because the current generated by flash EPROM cells and similar memorycells based on floating gates, is a strong function of the word linevoltage, applying a higher word line voltage to the reference cell thanto the selected cell in the memory array has the effect of inducingrelatively greater current flow in the reference bit line 24 than in thememory bit line 11. The erase verify mode expects to find a conductivestate of the memory cell. Therefore, the current on the memory bit line11 must be high enough to trip the sense amplifier 27 with the senseratio set by resistances R1 and R2. However, because the reference cellin the reference mini-array 23 is driven harder than the selected memorycell in the memory array 10, the selected memory cell must be relativelymore conductive in order to trip the sense circuitry 27. This provides amargin for erase verify without the need for, or in addition to,altering the ratio R1:R2.

During program verify, the voltage on word line 12 is energized to avalue higher than V_(DD). The voltage PVS on line 30 is enabled at alevel which is less than the voltage on the word line 12, while EVS andthe word line coupled to V_(DD) 25 are disabled. Thus, the selectedmemory cell in the memory array is driven harder than the memory cell inthe reference mini-array 23, tending to cause a relatively highercurrent flow on the bit line 11 than on the reference bit line 24.During program verify, the system expects to find a non-conductive stateof the selected memory cell. Thus, by driving the selected memory cell10 harder than the reference memory cell in the reference array 23, aprogram verify margin is established, again without, or in addition to,altering the ratio R1:R2.

A detailed description of an implementation of a preferred embodiment ofthe present invention is provided with respect to FIGS. 2-8. FIGS. 2 and3 illustrate the structure of the memory array 10 and the referencemini-array 23, respectively.

FIG. 2 illustrates the configuration of the flash EPROM circuit for usein a preferred embodiment of the present invention. The flash EPROMcircuit includes a drain-source-drain configuration where two columns ofmemory cells have sources coupled to a local virtual ground line 52. Thedrains of a left hand column of cells are coupled to a local bit line 50and the drains of the right hand column are coupled to local bit line51. Thus, for example, memory cell 53 has a source coupled to virtualground line 52 and a drain coupled to local bit line 50. Memory cell 54includes a source coupled to virtual ground line 52 and a drain coupledto local bit line 51. The gates of the memory cells are coupled to wordlines WL₀, WL₁, . . . , WL₃₁, for a 32 row high column. Thus, thecontrol gates of the floating gate transistors 53 and 54 are coupled toword line WL₁.

The local bit lines 50 and 51 are coupled through top block selecttransistors 55 and 56, respectively, through diffusion to metal contacts57 and 58 to global bit lines 59 and 60, respectively. The local virtualground line 52 is coupled through a bottom block select transistor 61 toa virtual ground line 62. The top block select transistors 55, 56 arecontrolled by the signal TBSEL on line 63 and the bottom block selecttransistors are controlled by the signal BBSEL on line 64.

The global bit lines are coupled through column select transistors 65and 66 as an input to the sense amps on lines 67. The column selecttransistors 65 and 66 are controlled by the output of the column decodercircuit 17, as shown in FIG. 1.

Each of the word lines WL₀ through WL₃₁ is driven by a word line driver,such as word line driver 22, shown in FIG. 1, such that the energizingvoltage applied to the word line can be any one off a set of particularvalues, depending on whether the chip is in the read, program, programverify, erase, or erase verify modes. Similarly, the virtual groundterminal on line 62 and the bit lines through line 67 are controlled independence on the mode of the circuit.

As can be seen, the bit lines 59 and 60 continue vertically to anotherpair of columns of cells which are a mirror image of the set shown inthe figure. Also, there are a plurality of sets of memory cell columnswhich make up the entire memory array. In the preferred system, theremay be one megabit or more of storage per device.

As shown in FIG. 3, the reference mini-array 23 is implemented withsimilar structures. Thus, the reference mini-array will include at leastone pair of columns of cells which include a local bit line 70 for theleft side and a local bit line 71 for the right side. The sources of theleft and right column memory cells are coupled to a local virtual groundline 72. The reference mini-array includes a column of 32 cell pairscoupled to word line WL₀ through WL₃₁. Top block select transistors 73and 74 are coupled to V_(DD) to establish an on state for both columns.The outputs BL0 and BL1 are coupled to respective reference bit linesfor supply to the sensing circuitry.

The bottom block select transistor 61 of FIG. 2 is not used in FIG. 3.Rather, the terminal of the local virtual ground line is coupled toground directly.

In normal read mode, the word line WL₁₅ is coupled to V_(DD) to energizecells 76 and 77 and generate a reference current for the normal readmode. During program verify, the word line WL₁₄ is coupled to the PVSvoltage to energize cells 78 and 79 and establish a reference currentfor program verify. During the erase verify mode, the word line WL₁₆ iscoupled to EVS to energize cells 80 and 81 and establish a referencecurrent for erase verify. The word lines of all of the other transistorsin the array, WL₀ through WL₁₃, and WL₁₇ through WL₃₁, are coupled toground.

FIGS. 4, 5, and 6 illustrate the circuitry used for generating the wordline voltages for a memory device using the cell structure of FIGS. 2and 3. In FIG. 4, an AVX generator is illustrated. The AVX generatorsupplies the AVX output on line 100 from a multiplexer 101. Themultiplexer receives as inputs the V_(DD) value on line 102, the V_(PP)value on line 103, and the output of an erase verify/program verifydriver on line 104. As noted in the figure, V_(DD) is passed on as theAVX level during normal read operations. Similarly, V_(PP) is passed onas the AVX level during program operations. During erase verify orprogram verify, the driver, generally 105, is used to establish thelevel of AVX. The driver 105 is controlled by the level of V_(PP) online 106. It comprises a controllable voltage source having a firstn-channel transistor 107 having its gate and drain coupled to terminal106 and its source coupled to the drain of second n-channel transistor108. The source of second n-channel 108 is coupled to the drain of thirdn-channel transistor 109. The source of n-channel transistor 109 iscoupled to node 110. Similarly, n-channel transistor 111, having itsgate and drain coupled to node 106, is included. The source oftransistor 111 is coupled to n-channel transistor 112. The source ofn-channel 112 is coupled to the drain of n-channel transistor 113. Thesource of n-channel 113 is coupled to node 110.

Node 110 is coupled to the drain of n-channel transistor 114, which hasits gate tied to V_(DD). The source of transistor 114 is coupled to node115. Node 115 is coupled to the drain of transistor 116 and the drain oftransistor 117. The sources of transistors 116 and 117 are coupled tonode 118. Node 118 is coupled to the drain of transistor 119 which hasits source coupled to ground.

During erase verify, control signal EVC is asserted at the gate oftransistor 108, and control signal VFYN is asserted at the gate oftransistor 119. The reference voltages EV1 and EV2 generated by themargin voltage generator 34 of FIG. 1 are applied to the gates oftransistors 109 and 116, respectively. Also during erase verify, thecontrol signal PVC and reference voltages PV1 and PV2 are disabled. Thisresults in generation of a particular voltage on line 104 which isdefined by the levels EV1 and EV2.

During program verify, the control signals PVC and VFYN are asserted andthe reference voltages PV1 and PV2 are applied to the gates oftransistors 113 and 117, respectively, and EV1, EV2, and EVC aredisabled. This results in the generation of a particular voltage on line104 for supply as AVX on line 100 during program verify defined by PV1and PV2.

The reference voltages EV1, EV2, EVC, PV1, PV2, and PVC are generated byvoltage margin generator 34. The voltage margin generator in thepreferred embodiment is illustrated in FIG. 5.

The voltage margin generator in FIG. 5 is based primarily on two voltagedividers composed of respective series of diode connected p-channeltransistors, formed in n-wells and having their sources coupled to then-well. Also, each voltage divider is driven by its respective controllogic.

Thus, the voltage divider for the program verify mode receives as inputsthe V_(PP) potential on line 200, the PGMVFY control signal on line 201which is asserted high during program verify mode, and the VPPH signalon line 202, which goes high to V_(DD) when the V_(PP) supply voltage200 goes high. Thus, the PGMVFY signal on line 201 is connected toinverter 203. The output of inverter 203 is connected to pass transistor206 which has its gate connected to V_(DD). The drain of pass transistor206 is supplied as input to a second control gate which is composed ofp-channel transistor 207, p-channel transistor 208, n-channel transistor209, and n-channel transistor 210, all of which are connected in series.The gates of transistors 207 and 210 are connected to the output oftransistor 206. The gate of transistor 209 is connected to V_(DD). Thegate of transistor 208 is connected to the VPPH line 202. Also, theoutput of transistor 206 is coupled to the drain of p-channel transistor211 which as its source coupled to V_(PP) and its gate connected to thenode between transistors 208 and 209.

The node 299 between transistors 208 and 209 is supplied out as the PVCcontrol signal, and gets pulled up to the V_(PP) value during programverify mode. Also, the node 299 is connected to a next inverter which iscomposed of p-channel transistor 212, p-channel transistor 213,n-channel transistor 214, and n-channel transistor 215 connected inseries. The node 299 is connected to the gates of transistors 212 and215. The gate of transistor 214 is coupled to V_(DD), and the gate ofp-channel transistor 213 is coupled to the VPPH signal on line 202.

As indicated by lines 216 and 217, the p-channel transistors 207, 208,212, 213 are formed in n-wells 216 and 217 which are coupled to theV_(PP) terminal 200.

Whether a V_(PP) is at high voltage or not, transistors 208 and 213 arealways on. However, when VPPH is at V_(DD), transistors 213 and 208 aremore resistive than when VPPH is low. Thus, these transistors provideprotection to the inverter during the high voltage transitions.

The resistive voltage divider which generates the reference voltagesPV1, PVS, and PV2 is enabled by the signal on the node 224 betweentransistors 213 and 214, which is pulled down during the program verifymode.

The voltage divider is comprised of p-channel transistors 218 through223. The gate of p-channel transistor 218 is connected to the signal online 224 at the node between transistors 213 and 214. The source oftransistor 218 is coupled to V_(PP) and to the n-well in which it isformed. P-channel transistors 219 through 223 are all diode connected,having their gates and drains connected together. Also, each of thetransistors is formed in a separate n-well coupled to its source.

The signal PV1 is generated on line 225 which is connected to the nodebetween transistors 219 and 220. N-channel transistors 226 and 227 arecoupled in series between line 225 and ground. The gate of transistor226 is coupled to V_(DD). The gate of transistor 227 is coupled tocontrol line 228.

The reference voltage PVS is generated on line 229 which is connected tothe node between transistors 221 and 222. Line 229 is also connected ton-channel transistors 230 and 231 connected in series to ground.N-channel transistor 230 has its gate coupled to V_(DD), and n-channeltransistor 231 has its gate connected to the control line 232.

The signal PV2 is generated on line 232. Line 232 is coupled to the nodebetween transistors 222 and 223. Also, line 232 is coupled to n-channeltransistor 233 which as its gate connected to control line 228 and itssource coupled to ground.

The control line 228 is derived from the PGMVFY signal on line 201through inverters 234, 235, and 236. Thus, control line 228 is low whenPGMVFY on line 201 is high. This enables the outputs PV1, PVS, and PV2for the program verify operation.

The voltage divider circuit for the erase verify mode has the samegeneral control logic, generally 250, as the voltage divider for theprogram verify mode. Thus, the control logic 250 will not be describedin the text. Of course, the control logic 250 is controlled by theERSVFY signal on line 251, rather than PGMVFY on line 201. The EVCsignal is asserted on line 253 during erase verify mode.

Also, the voltage divider for the erase verify controllable voltagesource is slightly different. It consists of p-channel transistors 254through 259, n-channel transistor 260, and p-channel transistor 261connected in series between V_(PP) at node 262 and ground at node 263.Transistor 254 has its gate connected to the output of the control logic250 which enables the voltage divider during erase verify mode. Thesource and n-well of transistor 254 are coupled to V_(PP) on line 262.The p-channel transistors 255 through 259 and 261 are diode connectedwith their gates and drains connected together and their sources coupledto the n-well in which they are formed, and connected in series.N-channel transistor 260 is diode connected With its gate coupled to itsdrain, and connected to the drain of p-channel transistor 259. Thesource of n-channel transistor 261 is coupled to the source of p-channeltransistor 261.

The voltage EV1 is generated on line 264 which is connected to the nodebetween transistors 257 and 258. Line 264 is also connected to n-channeltransistor 265 and 266 connected in series to ground. The gate ofn-channel transistors 265 is coupled to V_(DD), and the gate ofn-channel transistor 266 is coupled to the control line 267.

The reference voltage EVS is generated on line 268. Line 268 isconnected to the node between transistors 258 and 259. N-channeltransistors 269 and 270 are coupled in series between the line 268 andground. The gate of n-channel transistor 269 is coupled to V_(DD) andthe gate of n-channel transistor 270 is coupled to control line 267.

The reference voltage EV2 is generated on line 271. Line 271 isconnected to the node between transistors 260 and 259. N-channeltransistor 272 is connected between line 271 and ground. The gate oftransistor 272 is connected to the control line 267. Thus, the referencevoltages EV1, EVS, and EV2 are generated during erase verify mode.

For the typical case, V_(DD) is 5 V±0.5 V, for a possible range of 5.5 Vto 4.5 V, or ±10%. The typical case, the programming potential V_(PP) is12 V±0.6 V, for a range from 12.6 V to 11.4 V, or ±5%. Thus, the V_(PP)potential is a more tightly controlled value than V_(DD),percentage-wise. Using V_(PP) in the voltage margin generator results inmore carefully controlled reference voltages PV1, PVS, PV2, EV1, EVS,and EV2 than are possible on similar circuits based on V_(DD).

Thus, the levels for a preferred embodiment of the present invention ofthe signals involved in the circuits of FIGS. 4 and 5 are shown in thefollowing table along with the sense ratio SR based upon adjustment ofR1/R2 for the read mode, the erase verify EVFY mode, and the programverify PVFY mode.

    ______________________________________                                               READ      EVFY        PVFY                                             ______________________________________                                        AVX      5 V ± 10%                                                                              4.8 V ± 5%                                                                             6.5 V ± 5%                                EVS      0 V         5.05 V ± 5%                                                                            0 V                                          PVS      0 V         0 V         4.75 V ± 5%                               EV1      0 V         6.81 V ± 5%                                                                            0 V                                          EV2      0 V         3.1 V ± 5%                                                                             0 V                                          PV1      0 V         0 V         9 V ± 5%                                  PV2      0 V         0 V         2.8 V ± 5%                                EVC      0 V         12 V ± 5%                                                                              0 V                                          PVC      0 V         0 V         12 V ± 5%                                 VFYN     0 V         5 V         5 V                                          SR       2.5         1.5         2.5                                          ______________________________________                                    

The use of p-channel diode connected transistors having their n-wellstied to their sources provides a very small die area for the voltagedivider, and consumes a relatively small amount of power.

The use of n-channel transistor 261 for the controllable voltage sourcegenerating the erase verify references, compensates for the n-channelthreshold voltage variations of the AVX generator circuit shown in FIG.4. As a result, the voltage AVX generated by the circuit of FIG. 4 doesnot depend on the n-channel variation in threshold which arise due toprocess variations and the like.

FIG. 6 illustrates the implementation of the word line driver. There isa word line driver for each word line in the array, which is controlledby the row decoder shown in FIG. 1. The word line driver receives aninput XR from decoding circuitry on line 150, an enable signal IN online 151, and control signal VXP on line 158. The word line is driven online 152 at the output of a voltage level translation circuit.

The voltage level translator in FIG. 6 is composed of the inverterincluding p-channel transistor 153 and n-channel transistor 154 and passtransistor 157. Pass transistor 157 has its source connected to theinput terminal 151 and its drain connected to line 156. The gate oftransistor 157 receives the control signal XR on line 150 from thedecoding circuitry. The gates of transistors 153 and 154 are connectedto line 156. The word line 152 is connected to the node betweentransistors 153 and 154. The source of transistor 154 is connected toground and the source of transistor 153 is connected to the AVX line100. Also, the n-well in which the p-channel transistor 153 is formed iscoupled to the AVX line 100.

P-channel transistor 155 has its drain connected to line 156 and itssource connected to the AVX terminal. Transistor 155 serves as a ratiopull up transistor for a NAND gate driver which supplies the signal INon line 151. The n-well in which the transistor 155 is formed is alsoconnected to the AVX line 100. The gate of transistor 155 is connectedto control signal VXP on line 158 which serves to regulate the strengthof pull up transistor 155.

Thus, the level translator shown in FIG. 6 supplies substantially thevoltage AVX on the word line 152 when enabled by an input signal atsubstantially the V_(DD) level. Because of the more careful control ofthe V_(PP) potential, the AVX potential is also more carefullycontrolled, resulting in a word line voltage during the erase andprogram verify modes with more tightly specified values, as explainedabove.

FIG. 7 illustrates a preferred embodiment of the reference mini-array 23for the system employing cells of FIG. 2. The reference mini-array shownin FIG. 7 includes three pairs of columns of cells 300, 301, and 302.Each pair, 300, 301, 302, is implemented as shown in FIG. 3. The columnpair 300 has its bit lines coupled to ground. The column pair 302 hasits bit lines coupled to ground. The column pair 301 drives SFL and SFRlines 307 and 308 used as reference for sensing circuitry. The SFL andSFR bit lines 307, 308 are connected to lines corresponding to BL0 andBL1 of FIG. 3.

As can be seen, word lines 14, 15, and 16 of the blocks of referencecells are coupled to the PVS, V_(DD), and EVS values, respectively. TheV_(DD) value is generated on line 303 at the output of inverter 304.Inverter 304 is driven by inverter 305. Inverter 305 is driven by NORgate 306. NOR gate 306 receives as inputs the PGMVFY and ERSVFY signals.Therefore, the signal on line 303 is driven with V_(DD) if the circuitis in the read mode. Otherwise, the signal on line 303 is substantiallygrounded. The PVS and EVS signals are controlled by the voltage margincircuit shown in FIG. 4 as described above.

The reference array shown drives three reference cells with particularenergizing voltages. Alternative systems may multiplex the three levelsto the word line of a single reference cell, or another combination ofcells as suits a designer.

The SFL and SFR bit lines 307 and 308 are coupled to a reference bitline load as shown in FIG. 8, through a least significant bit decodeenabling one or the other. The load circuitry, generally 408, shown inFIG. 8, is designed using mask options so that an integrated circuitincorporating the load circuit 408 may be easily modified to achieve avariety of effects.

According to one mask option, the resistance R2 affected by the loadcircuitry 408 is constant in the erase verify, program verify, and readmodes. According to other mask options, the ability to change theresistance R2 is provided to adjust the sense ratio.

The load circuitry 408 consists of transistors 420 through 425, as shownin the figure, having the widths and lengths shown by way of example.Four mask options 426, 427, 428, and 429 are designed into thecircuitry.

Mask option 428 is closed to provide for adjustment of the sense ratioduring erase verify. If opened, mask option 428 connects ground to theinput of NOR gate 432, so that no adjustment is made during eraseverify. Mask option 429 is left open if the sense ratio is to remainconstant during the program verify mode. Otherwise, the mask option 429is closed to allow adjustment. Mask options 426 and 427 are used foradjustments of the sense ratio in general.

Thus, in the mode illustrated where mask option 429 is open, transistors420 through 424 are involved in the sense ratio parameter. Transistor420 is coupled between node 411 and V_(DD). Its gate is driven by theoutput of inverter 430, which receives the OVER signal on line 431 asinput. Transistors 423, 422, and 421 are connected in series between thenode 411 and V_(DD). Their gates are connected together and to theterminal V_(DD). According to mask option 426, transistor 421 isbypassed so that only transistors 422 and 423 contribute to the loadresistance. By opening mask option 426, this resistance can beincreased. Similarly, by closing mask option 427, this resistance can bedecreased.

Transistor 424 is connected between terminal 411 and V_(DD). Its gate isdriven by the output of NOR gate 432. The inputs to NOR gate 432 includethe OVER signal on line 431, and, depending on mask option 428, eitherGND or an erase verify control signal ERSVFY.

The OVER signal is only asserted during an over erase checking mode.Thus, it is normally low, which turns on transistors 420 and 424 so thatthey participate in the sense ratio resistance. The sense ratio in thepreferred system is about 2.5 in this condition. It may be adjusted upto about 3 and down to about 2.33 using the mask options 426 and 427, asmentioned above.

With the mask option 428 connected as shown, then the sense ratio duringerase verify is adjusted by disabling transistor 424 to increase theimpedance. This has the effect of lowering the sense ratio to about 1.5.Similarly, during program verify, mask option 429 may be connected sothat transistor 425 is enabled by the PGMVFY control signal on line 433.This has the effect of reducing the impedance and increasing the senseratio to about 4.0 during the program verify mode.

The sizes of the transistors are provided in FIG. 8 by way of example.It will be appreciated by those of skill in the art that the resistanceof the load circuitry 408 may be manipulated by manipulating the widthsand lengths of the MOS transistors involved.

In summary, the present invention provides the program verify and eraseverify margins by applying different potentials from a V_(PP) divider tothe word lines in the memory array than to the word lines in thereference array, during the program and erase verify modes. Because themargins are a strong function of the difference in potential on the wordlines, the margins can be easily adjusted and built-in to a specificdesign without varying the sense ratio. Furthermore, because the marginsare determined using a margin generator responsive to the more tightlycontrolled programming potential V_(PP), better performance is achievedcompared to prior art designs which generate the word line potentialsduring these modes based on the less tightly controlled supply voltageV_(DD). Furthermore, the margin voltage generator 34 of the presentinvention is implemented using a unique structure using a small diearea, low power and which compensates for threshold variations which mayaffect the ultimate values generated by the circuit.

Therefore, a non-volatile integrated circuit memory device is providedwith erase verify and program verify modes that can be more preciselycontrolled than prior art systems.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in this art.The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

What is claimed is:
 1. For a non-volatile memory device withprogrammable memory cells having control terminals, and a reference cellhaving a control terminal, and sense circuitry responsive to addressedprogrammable memory cells and the reference cell, and having a readpotential supplied to the control terminals of the programmable memorycells and a reference potential supplied to the control terminal of thereference cell during a read mode, a method for verifying a state of theprogrammable memory cells, comprising:supplying a first verify potentialto the control terminal of an addressed programmable memory cell,supplying a second verify potential to the control terminal of thereference cell different from the first verify potential; and sensingthe state of the addressed programmable memory cell with the sensecircuitry during said supplying steps.
 2. The method of claim 1, whereinthe reference potential and the read potential are substantially equalduring the read mode.
 3. The method of claim 1, for verifying a state ofthe programmable memory cells which is non-conductive in response to theread potential applied to the control terminal of the programmablememory cells, wherein the second verify potential is less than the firstverify potential.
 4. The method of claim 1 for verifying a state of theprogrammable memory cells which is conductive in response to the readpotential applied to the control terminals of the programmable memorycells, wherein the second verify potential is greater than the firstverify potential.
 5. The method of claim 3, wherein the first verifypotential is greater than the read potential.
 6. The method of claim 4,wherein the first verify potential is less than the read potential. 7.The method of claim 1, wherein the sense circuitry includes a referencebit line coupled to the reference cell, the reference bit line having acharacteristic load during the read mode, and including:increasing theload on the reference bit line during the step of supplying the secondverify potential to the control terminal of the reference cell.